The galvanization of the exterior surface of pipe or conduit as part of the continuous manufacture thereof from an endless strip of sheet metal has been practiced commercially for a number of years. The process basically consists of roll-forming the metal strip into tubular form after drawing it from an endless supply, welding the seam, scarfing and dressing off the weld, and passing the continuously formed tube through a pickling bath and rinse. The tube is then passed through a preheating station and then through a bath of molten zinc, after which the excess zinc is removed, the tube cooled to handling temperature in a water bath, and the tube sheared into finite lengths. The tube may be subjected to a sizing operation after being cooled, prior to the shearing operation.
Such an integrated continuous manufacturing process is disclosed, for example, in U.S. Pat. No. 3,226,817, with particular emphasis on the galvanization step of the process in U.S. Pat. Nos. 3,226,817, 3,259,148, and 3,877,975.
In the galvanizing stations of such prior integrated processes, the continuously-formed, rapidly moving tube, after appropriate preparation, was passed through an elongated trough positioned above a pool of molten zinc in a large vat, from which a stream of the liquid metal was pumped to maintain a substantial and overflowing body of molten zinc in the trough as well as to replace the zinc being carried away from the trough as a fluid coating on the tube.
As described in co-pending application Ser. No. 07/892,432, filed Jun. 10, 1992, assigned to the same assignee as the present application, it has recently been found that coating of linear elements in a continuous galvanizing process may be effected by immersion of the linear elements in molten zinc in an open-ended tube, to which the zinc is pumped with zinc flowing out of the opposite ends of the tube. This arrangement enables galvanizing to be accomplished with reduced zinc flow as compared with prior methods employing overflowing troughs. Reduction of zinc flow is generally desirable due to the consequent reduction of the corrosive and abrasive effects of molten zinc on the zinc pump and other system components.
In a further development described and claim in subsequent U.S. patent application Ser. No. 08/026,432, filed Mar. 4, 1993, now U.S. Pat. No. 5,364,661, issued Nov. 15, 1994 and assigned to the same assignee as the present application molten zinc was applied to the linear element, specifically a tube, by passing it concentrically through a surrounding and converging conical curtain of flowing molten zinc issuing from the orifice of an encircling nozzle.
More recently, the practicality of galvanizing linear elements by simply passing them through a transversely flowing stream of molten zinc has been established.